Anthrax is a highly lethal infection caused by infection with the Gram-positive bacterium, Bacillus anthracis. In naturally-acquired cases, organisms usually gain entrance through skin wounds (causing a localized infection), but may be inhaled or ingested. Intentional release by belligerents or terrorist groups would presumably involve the aerosol route, as the spore form of the bacillus is quite stable and possess characteristics ideal for the generation of aerosols.
There is extensive coverage of the DOD program to vaccinate American soldiers against anthrax.